/**
 * @file
 * Network Point to Point Protocol over Layer 2 Tunneling Protocol program file.
 *
 */

/*
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
 * OF SUCH DAMAGE.
 *
 * This file is part of the lwIP TCP/IP stack.
 *
 */

/*
 * L2TP Support status:
 *
 * Supported:
 * - L2TPv2 (PPP over L2TP, a.k.a. UDP tunnels)
 * - LAC
 *
 * Not supported:
 * - LNS (require PPP server support)
 * - L2TPv3 ethernet pseudowires
 * - L2TPv3 VLAN pseudowire
 * - L2TPv3 PPP pseudowires
 * - L2TPv3 IP encapsulation
 * - L2TPv3 IP pseudowire
 * - L2TP tunnel switching - http://tools.ietf.org/html/draft-ietf-l2tpext-tunnel-switching-08
 * - Multiple tunnels per UDP socket, as well as multiple sessions per tunnel
 * - Hidden AVPs
 */

#include "netif/ppp/ppp_opts.h"
#if PPP_SUPPORT && PPPOL2TP_SUPPORT /* don't build if not configured for use in lwipopts.h */

#include "lwip/err.h"
#include "lwip/memp.h"
#include "lwip/netif.h"
#include "lwip/udp.h"
#include "lwip/snmp.h"

#include "netif/ppp/ppp_impl.h"
#include "netif/ppp/lcp.h"
#include "netif/ppp/ipcp.h"
#include "netif/ppp/pppol2tp.h"
#include "netif/ppp/pppcrypt.h"
#include "netif/ppp/magic.h"

/* Memory pool */
LWIP_MEMPOOL_DECLARE(PPPOL2TP_PCB, MEMP_NUM_PPPOL2TP_INTERFACES, sizeof(pppol2tp_pcb), "PPPOL2TP_PCB")

/* callbacks called from PPP core */
static err_t pppol2tp_write(ppp_pcb *ppp, void *ctx, struct pbuf *p);
static err_t pppol2tp_netif_output(ppp_pcb *ppp, void *ctx, struct pbuf *p, u_short protocol);
static err_t pppol2tp_destroy(ppp_pcb *ppp, void *ctx);    /* Destroy a L2TP control block */
static void pppol2tp_connect(ppp_pcb *ppp, void *ctx);    /* Be a LAC, connect to a LNS. */
static void pppol2tp_disconnect(ppp_pcb *ppp, void *ctx);  /* Disconnect */

/* Prototypes for procedures local to this file. */
static void pppol2tp_input(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr, u16_t port);
static void pppol2tp_dispatch_control_packet(pppol2tp_pcb *l2tp, u16_t port, struct pbuf *p, u16_t ns, u16_t nr);
static void pppol2tp_timeout(void *arg);
static void pppol2tp_abort_connect(pppol2tp_pcb *l2tp);
static err_t pppol2tp_send_sccrq(pppol2tp_pcb *l2tp);
static err_t pppol2tp_send_scccn(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_send_icrq(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_send_iccn(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_send_zlb(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_send_stopccn(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_xmit(pppol2tp_pcb *l2tp, struct pbuf *pb);
static err_t pppol2tp_udp_send(pppol2tp_pcb *l2tp, struct pbuf *pb);

/* Callbacks structure for PPP core */
static const struct link_callbacks pppol2tp_callbacks =
{
    pppol2tp_connect,
#if PPP_SERVER
    NULL,
#endif /* PPP_SERVER */
    pppol2tp_disconnect,
    pppol2tp_destroy,
    pppol2tp_write,
    pppol2tp_netif_output,
    NULL,
    NULL
};


/* Create a new L2TP session. */
ppp_pcb *pppol2tp_create(struct netif *pppif,
                         struct netif *netif, const ip_addr_t *ipaddr, u16_t port,
                         const u8_t *secret, u8_t secret_len,
                         ppp_link_status_cb_fn link_status_cb, void *ctx_cb)
{
    ppp_pcb *ppp;
    pppol2tp_pcb *l2tp;
    struct udp_pcb *udp;
#if !PPPOL2TP_AUTH_SUPPORT
    LWIP_UNUSED_ARG(secret);
    LWIP_UNUSED_ARG(secret_len);
#endif /* !PPPOL2TP_AUTH_SUPPORT */

    if (ipaddr == NULL)
    {
        goto ipaddr_check_failed;
    }

    l2tp = (pppol2tp_pcb *)LWIP_MEMPOOL_ALLOC(PPPOL2TP_PCB);
    if (l2tp == NULL)
    {
        goto memp_malloc_l2tp_failed;
    }

    udp = udp_new_ip_type(IP_GET_TYPE(ipaddr));
    if (udp == NULL)
    {
        goto udp_new_failed;
    }
    udp_recv(udp, pppol2tp_input, l2tp);

    ppp = ppp_new(pppif, &pppol2tp_callbacks, l2tp, link_status_cb, ctx_cb);
    if (ppp == NULL)
    {
        goto ppp_new_failed;
    }

    memset(l2tp, 0, sizeof(pppol2tp_pcb));
    l2tp->phase = PPPOL2TP_STATE_INITIAL;
    l2tp->ppp = ppp;
    l2tp->udp = udp;
    l2tp->netif = netif;
    ip_addr_copy(l2tp->remote_ip, *ipaddr);
    l2tp->remote_port = port;
#if PPPOL2TP_AUTH_SUPPORT
    l2tp->secret = secret;
    l2tp->secret_len = secret_len;
#endif /* PPPOL2TP_AUTH_SUPPORT */

    return ppp;

ppp_new_failed:
    udp_remove(udp);
udp_new_failed:
    LWIP_MEMPOOL_FREE(PPPOL2TP_PCB, l2tp);
memp_malloc_l2tp_failed:
ipaddr_check_failed:
    return NULL;
}

/* Called by PPP core */
static err_t pppol2tp_write(ppp_pcb *ppp, void *ctx, struct pbuf *p)
{
    pppol2tp_pcb *l2tp = (pppol2tp_pcb *)ctx;
    struct pbuf *ph; /* UDP + L2TP header */
    err_t ret;
#if MIB2_STATS
    u16_t tot_len;
#else /* MIB2_STATS */
    LWIP_UNUSED_ARG(ppp);
#endif /* MIB2_STATS */

    ph = pbuf_alloc(PBUF_TRANSPORT, (u16_t)(PPPOL2TP_OUTPUT_DATA_HEADER_LEN), PBUF_RAM);
    if(!ph)
    {
        LINK_STATS_INC(link.memerr);
        LINK_STATS_INC(link.proterr);
        MIB2_STATS_NETIF_INC(ppp->netif, ifoutdiscards);
        pbuf_free(p);
        return ERR_MEM;
    }

    pbuf_header(ph, -(s16_t)PPPOL2TP_OUTPUT_DATA_HEADER_LEN); /* hide L2TP header */
    pbuf_cat(ph, p);
#if MIB2_STATS
    tot_len = ph->tot_len;
#endif /* MIB2_STATS */

    ret = pppol2tp_xmit(l2tp, ph);
    if (ret != ERR_OK)
    {
        LINK_STATS_INC(link.err);
        MIB2_STATS_NETIF_INC(ppp->netif, ifoutdiscards);
        return ret;
    }

    MIB2_STATS_NETIF_ADD(ppp->netif, ifoutoctets, (u16_t)tot_len);
    MIB2_STATS_NETIF_INC(ppp->netif, ifoutucastpkts);
    LINK_STATS_INC(link.xmit);
    return ERR_OK;
}

/* Called by PPP core */
static err_t pppol2tp_netif_output(ppp_pcb *ppp, void *ctx, struct pbuf *p, u_short protocol)
{
    pppol2tp_pcb *l2tp = (pppol2tp_pcb *)ctx;
    struct pbuf *pb;
    u8_t *pl;
    err_t err;
#if MIB2_STATS
    u16_t tot_len;
#else /* MIB2_STATS */
    LWIP_UNUSED_ARG(ppp);
#endif /* MIB2_STATS */

    /* @todo: try to use pbuf_header() here! */
    pb = pbuf_alloc(PBUF_TRANSPORT, PPPOL2TP_OUTPUT_DATA_HEADER_LEN + sizeof(protocol), PBUF_RAM);
    if(!pb)
    {
        LINK_STATS_INC(link.memerr);
        LINK_STATS_INC(link.proterr);
        MIB2_STATS_NETIF_INC(ppp->netif, ifoutdiscards);
        return ERR_MEM;
    }

    pbuf_header(pb, -(s16_t)PPPOL2TP_OUTPUT_DATA_HEADER_LEN);

    pl = (u8_t *)pb->payload;
    PUTSHORT(protocol, pl);

    pbuf_chain(pb, p);
#if MIB2_STATS
    tot_len = pb->tot_len;
#endif /* MIB2_STATS */

    if( (err = pppol2tp_xmit(l2tp, pb)) != ERR_OK)
    {
        LINK_STATS_INC(link.err);
        MIB2_STATS_NETIF_INC(ppp->netif, ifoutdiscards);
        return err;
    }

    MIB2_STATS_NETIF_ADD(ppp->netif, ifoutoctets, tot_len);
    MIB2_STATS_NETIF_INC(ppp->netif, ifoutucastpkts);
    LINK_STATS_INC(link.xmit);
    return ERR_OK;
}

/* Destroy a L2TP control block */
static err_t pppol2tp_destroy(ppp_pcb *ppp, void *ctx)
{
    pppol2tp_pcb *l2tp = (pppol2tp_pcb *)ctx;
    LWIP_UNUSED_ARG(ppp);

    sys_untimeout(pppol2tp_timeout, l2tp);
    udp_remove(l2tp->udp);
    LWIP_MEMPOOL_FREE(PPPOL2TP_PCB, l2tp);
    return ERR_OK;
}

/* Be a LAC, connect to a LNS. */
static void pppol2tp_connect(ppp_pcb *ppp, void *ctx)
{
    err_t err;
    pppol2tp_pcb *l2tp = (pppol2tp_pcb *)ctx;
    lcp_options *lcp_wo;
    lcp_options *lcp_ao;
#if PPP_IPV4_SUPPORT && VJ_SUPPORT
    ipcp_options *ipcp_wo;
    ipcp_options *ipcp_ao;
#endif /* PPP_IPV4_SUPPORT && VJ_SUPPORT */

    l2tp->tunnel_port = l2tp->remote_port;
    l2tp->our_ns = 0;
    l2tp->peer_nr = 0;
    l2tp->peer_ns = 0;
    l2tp->source_tunnel_id = 0;
    l2tp->remote_tunnel_id = 0;
    l2tp->source_session_id = 0;
    l2tp->remote_session_id = 0;
    /* l2tp->*_retried are cleared when used */

    lcp_wo = &ppp->lcp_wantoptions;
    lcp_wo->mru = PPPOL2TP_DEFMRU;
    lcp_wo->neg_asyncmap = 0;
    lcp_wo->neg_pcompression = 0;
    lcp_wo->neg_accompression = 0;
    lcp_wo->passive = 0;
    lcp_wo->silent = 0;

    lcp_ao = &ppp->lcp_allowoptions;
    lcp_ao->mru = PPPOL2TP_DEFMRU;
    lcp_ao->neg_asyncmap = 0;
    lcp_ao->neg_pcompression = 0;
    lcp_ao->neg_accompression = 0;

#if PPP_IPV4_SUPPORT && VJ_SUPPORT
    ipcp_wo = &ppp->ipcp_wantoptions;
    ipcp_wo->neg_vj = 0;
    ipcp_wo->old_vj = 0;

    ipcp_ao = &ppp->ipcp_allowoptions;
    ipcp_ao->neg_vj = 0;
    ipcp_ao->old_vj = 0;
#endif /* PPP_IPV4_SUPPORT && VJ_SUPPORT */

    /* Listen to a random source port, we need to do that instead of using udp_connect()
     * because the L2TP LNS might answer with its own random source port (!= 1701)
     */
#if LWIP_IPV6
    if (IP_IS_V6_VAL(l2tp->udp->local_ip))
    {
        udp_bind(l2tp->udp, IP6_ADDR_ANY, 0);
    }
    else
#endif /* LWIP_IPV6 */
        udp_bind(l2tp->udp, IP_ADDR_ANY, 0);

#if PPPOL2TP_AUTH_SUPPORT
    /* Generate random vector */
    if (l2tp->secret != NULL)
    {
        magic_random_bytes(l2tp->secret_rv, sizeof(l2tp->secret_rv));
    }
#endif /* PPPOL2TP_AUTH_SUPPORT */

    do
    {
        l2tp->remote_tunnel_id = magic();
    }
    while(l2tp->remote_tunnel_id == 0);
    /* save state, in case we fail to send SCCRQ */
    l2tp->sccrq_retried = 0;
    l2tp->phase = PPPOL2TP_STATE_SCCRQ_SENT;
    if ((err = pppol2tp_send_sccrq(l2tp)) != 0)
    {
        PPPDEBUG(LOG_DEBUG, ("pppol2tp: failed to send SCCRQ, error=%d\n", err));
    }
    sys_timeout(PPPOL2TP_CONTROL_TIMEOUT, pppol2tp_timeout, l2tp);
}

/* Disconnect */
static void pppol2tp_disconnect(ppp_pcb *ppp, void *ctx)
{
    pppol2tp_pcb *l2tp = (pppol2tp_pcb *)ctx;

    l2tp->our_ns++;
    pppol2tp_send_stopccn(l2tp, l2tp->our_ns);

    /* stop any timer, disconnect can be called while initiating is in progress */
    sys_untimeout(pppol2tp_timeout, l2tp);
    l2tp->phase = PPPOL2TP_STATE_INITIAL;
    ppp_link_end(ppp); /* notify upper layers */
}

/* UDP Callback for incoming IPv4 L2TP frames */
static void pppol2tp_input(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr, u16_t port)
{
    pppol2tp_pcb *l2tp = (pppol2tp_pcb *)arg;
    u16_t hflags, hlen, len = 0, tunnel_id = 0, session_id = 0, ns = 0, nr = 0, offset = 0;
    u8_t *inp;
    LWIP_UNUSED_ARG(pcb);

    /* we can't unbound a UDP pcb, thus we can still receive UDP frames after the link is closed */
    if (l2tp->phase < PPPOL2TP_STATE_SCCRQ_SENT)
    {
        goto free_and_return;
    }

    if (!ip_addr_cmp(&l2tp->remote_ip, addr))
    {
        goto free_and_return;
    }

    /* discard packet if port mismatch, but only if we received a SCCRP */
    if (l2tp->phase > PPPOL2TP_STATE_SCCRQ_SENT && l2tp->tunnel_port != port)
    {
        goto free_and_return;
    }

    /* printf("-----------\nL2TP INPUT, %d\n", p->len); */

    /* L2TP header */
    if (p->len < sizeof(hflags) + sizeof(tunnel_id) + sizeof(session_id) )
    {
        goto packet_too_short;
    }

    inp = (u8_t *)p->payload;
    GETSHORT(hflags, inp);

    if (hflags & PPPOL2TP_HEADERFLAG_CONTROL)
    {
        /* check mandatory flags for a control packet */
        if ( (hflags & PPPOL2TP_HEADERFLAG_CONTROL_MANDATORY) != PPPOL2TP_HEADERFLAG_CONTROL_MANDATORY )
        {
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: mandatory header flags for control packet not set\n"));
            goto free_and_return;
        }
        /* check forbidden flags for a control packet */
        if (hflags & PPPOL2TP_HEADERFLAG_CONTROL_FORBIDDEN)
        {
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: forbidden header flags for control packet found\n"));
            goto free_and_return;
        }
    }
    else
    {
        /* check mandatory flags for a data packet */
        if ( (hflags & PPPOL2TP_HEADERFLAG_DATA_MANDATORY) != PPPOL2TP_HEADERFLAG_DATA_MANDATORY)
        {
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: mandatory header flags for data packet not set\n"));
            goto free_and_return;
        }
    }

    /* Expected header size  */
    hlen = sizeof(hflags) + sizeof(tunnel_id) + sizeof(session_id);
    if (hflags & PPPOL2TP_HEADERFLAG_LENGTH)
    {
        hlen += sizeof(len);
    }
    if (hflags & PPPOL2TP_HEADERFLAG_SEQUENCE)
    {
        hlen += sizeof(ns) + sizeof(nr);
    }
    if (hflags & PPPOL2TP_HEADERFLAG_OFFSET)
    {
        hlen += sizeof(offset);
    }
    if (p->len < hlen)
    {
        goto packet_too_short;
    }

    if (hflags & PPPOL2TP_HEADERFLAG_LENGTH)
    {
        GETSHORT(len, inp);
        if (p->len < len || len < hlen)
        {
            goto packet_too_short;
        }
    }
    GETSHORT(tunnel_id, inp);
    GETSHORT(session_id, inp);
    if (hflags & PPPOL2TP_HEADERFLAG_SEQUENCE)
    {
        GETSHORT(ns, inp);
        GETSHORT(nr, inp);
    }
    if (hflags & PPPOL2TP_HEADERFLAG_OFFSET)
    {
        GETSHORT(offset, inp)
        if (offset > 4096)   /* don't be fooled with large offset which might overflow hlen */
        {
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: strange packet received, offset=%d\n", offset));
            goto free_and_return;
        }
        hlen += offset;
        if (p->len < hlen)
        {
            goto packet_too_short;
        }
        INCPTR(offset, inp);
    }

    /* printf("HLEN = %d\n", hlen); */

    /* skip L2TP header */
    if (pbuf_header(p, -(s16_t)hlen) != 0)
    {
        goto free_and_return;
    }

    /* printf("LEN=%d, TUNNEL_ID=%d, SESSION_ID=%d, NS=%d, NR=%d, OFFSET=%d\n", len, tunnel_id, session_id, ns, nr, offset); */
    PPPDEBUG(LOG_DEBUG, ("pppol2tp: input packet, len=%"U16_F", tunnel=%"U16_F", session=%"U16_F", ns=%"U16_F", nr=%"U16_F"\n",
                         len, tunnel_id, session_id, ns, nr));

    /* Control packet */
    if (hflags & PPPOL2TP_HEADERFLAG_CONTROL)
    {
        pppol2tp_dispatch_control_packet(l2tp, port, p, ns, nr);
        goto free_and_return;
    }

    /* Data packet */
    if(l2tp->phase != PPPOL2TP_STATE_DATA)
    {
        goto free_and_return;
    }
    if(tunnel_id != l2tp->remote_tunnel_id)
    {
        PPPDEBUG(LOG_DEBUG, ("pppol2tp: tunnel ID mismatch, assigned=%d, received=%d\n", l2tp->remote_tunnel_id, tunnel_id));
        goto free_and_return;
    }
    if(session_id != l2tp->remote_session_id)
    {
        PPPDEBUG(LOG_DEBUG, ("pppol2tp: session ID mismatch, assigned=%d, received=%d\n", l2tp->remote_session_id, session_id));
        goto free_and_return;
    }
    /*
     * skip address & flags if necessary
     *
     * RFC 2661 does not specify whether the PPP frame in the L2TP payload should
     * have a HDLC header or not. We handle both cases for compatibility.
     */
    if (p->len >= 2)
    {
        GETSHORT(hflags, inp);
        if (hflags == 0xff03)
        {
            pbuf_header(p, -(s16_t)2);
        }
    }
    /* Dispatch the packet thereby consuming it. */
    ppp_input(l2tp->ppp, p);
    return;

packet_too_short:
    PPPDEBUG(LOG_DEBUG, ("pppol2tp: packet too short: %d\n", p->len));
free_and_return:
    pbuf_free(p);
}

/* L2TP Control packet entry point */
static void pppol2tp_dispatch_control_packet(pppol2tp_pcb *l2tp, u16_t port, struct pbuf *p, u16_t ns, u16_t nr)
{
    u8_t *inp;
    u16_t avplen, avpflags, vendorid, attributetype, messagetype = 0;
    err_t err;
#if PPPOL2TP_AUTH_SUPPORT
    lwip_md5_context md5_ctx;
    u8_t md5_hash[16];
    u8_t challenge_id = 0;
#endif /* PPPOL2TP_AUTH_SUPPORT */

    l2tp->peer_nr = nr;
    l2tp->peer_ns = ns;
    /* printf("L2TP CTRL INPUT, ns=%d, nr=%d, len=%d\n", ns, nr, p->len); */

    /* Handle the special case of the ICCN acknowledge */
    if (l2tp->phase == PPPOL2TP_STATE_ICCN_SENT && l2tp->peer_nr > l2tp->our_ns)
    {
        l2tp->phase = PPPOL2TP_STATE_DATA;
    }

    /* ZLB packets */
    if (p->tot_len == 0)
    {
        return;
    }

    p = ppp_singlebuf(p);
    inp = (u8_t *)p->payload;
    /* Decode AVPs */
    while (p->len > 0)
    {
        if (p->len < sizeof(avpflags) + sizeof(vendorid) + sizeof(attributetype) )
        {
            goto packet_too_short;
        }
        GETSHORT(avpflags, inp);
        avplen = avpflags & PPPOL2TP_AVPHEADERFLAG_LENGTHMASK;
        /* printf("AVPLEN = %d\n", avplen); */
        if (p->len < avplen || avplen < sizeof(avpflags) + sizeof(vendorid) + sizeof(attributetype))
        {
            goto packet_too_short;
        }
        GETSHORT(vendorid, inp);
        GETSHORT(attributetype, inp);
        avplen -= sizeof(avpflags) + sizeof(vendorid) + sizeof(attributetype);

        /* Message type must be the first AVP */
        if (messagetype == 0)
        {
            if (attributetype != 0 || vendorid != 0 || avplen != sizeof(messagetype) )
            {
                PPPDEBUG(LOG_DEBUG, ("pppol2tp: message type must be the first AVP\n"));
                return;
            }
            GETSHORT(messagetype, inp);
            /* printf("Message type = %d\n", messagetype); */
            switch(messagetype)
            {
            /* Start Control Connection Reply */
            case PPPOL2TP_MESSAGETYPE_SCCRP:
                /* Only accept SCCRP packet if we sent a SCCRQ */
                if (l2tp->phase != PPPOL2TP_STATE_SCCRQ_SENT)
                {
                    goto send_zlb;
                }
                break;
            /* Incoming Call Reply */
            case PPPOL2TP_MESSAGETYPE_ICRP:
                /* Only accept ICRP packet if we sent a IRCQ */
                if (l2tp->phase != PPPOL2TP_STATE_ICRQ_SENT)
                {
                    goto send_zlb;
                }
                break;
            /* Stop Control Connection Notification */
            case PPPOL2TP_MESSAGETYPE_STOPCCN:
                pppol2tp_send_zlb(l2tp, l2tp->our_ns); /* Ack the StopCCN before we switch to down state */
                if (l2tp->phase < PPPOL2TP_STATE_DATA)
                {
                    pppol2tp_abort_connect(l2tp);
                }
                else if (l2tp->phase == PPPOL2TP_STATE_DATA)
                {
                    /* Don't disconnect here, we let the LCP Echo/Reply find the fact
                     * that PPP session is down. Asking the PPP stack to end the session
                     * require strict checking about the PPP phase to prevent endless
                     * disconnection loops.
                     */
                }
                return;
            default:
                break;
            }
            goto nextavp;
        }

        /* Skip proprietary L2TP extensions */
        if (vendorid != 0)
        {
            goto skipavp;
        }

        switch (messagetype)
        {
        /* Start Control Connection Reply */
        case PPPOL2TP_MESSAGETYPE_SCCRP:
            switch (attributetype)
            {
            case PPPOL2TP_AVPTYPE_TUNNELID:
                if (avplen != sizeof(l2tp->source_tunnel_id) )
                {
                    PPPDEBUG(LOG_DEBUG, ("pppol2tp: AVP Assign tunnel ID length check failed\n"));
                    return;
                }
                GETSHORT(l2tp->source_tunnel_id, inp);
                PPPDEBUG(LOG_DEBUG, ("pppol2tp: Assigned tunnel ID %"U16_F"\n", l2tp->source_tunnel_id));
                goto nextavp;
#if PPPOL2TP_AUTH_SUPPORT
            case PPPOL2TP_AVPTYPE_CHALLENGE:
                if (avplen == 0)
                {
                    PPPDEBUG(LOG_DEBUG, ("pppol2tp: Challenge length check failed\n"));
                    return;
                }
                if (l2tp->secret == NULL)
                {
                    PPPDEBUG(LOG_DEBUG, ("pppol2tp: Received challenge from peer and no secret key available\n"));
                    pppol2tp_abort_connect(l2tp);
                    return;
                }
                /* Generate hash of ID, secret, challenge */
                lwip_md5_init(&md5_ctx);
                lwip_md5_starts(&md5_ctx);
                challenge_id = PPPOL2TP_MESSAGETYPE_SCCCN;
                lwip_md5_update(&md5_ctx, &challenge_id, 1);
                lwip_md5_update(&md5_ctx, l2tp->secret, l2tp->secret_len);
                lwip_md5_update(&md5_ctx, inp, avplen);
                lwip_md5_finish(&md5_ctx, l2tp->challenge_hash);
                lwip_md5_free(&md5_ctx);
                l2tp->send_challenge = 1;
                goto skipavp;
            case PPPOL2TP_AVPTYPE_CHALLENGERESPONSE:
                if (avplen != PPPOL2TP_AVPTYPE_CHALLENGERESPONSE_SIZE)
                {
                    PPPDEBUG(LOG_DEBUG, ("pppol2tp: AVP Challenge Response length check failed\n"));
                    return;
                }
                /* Generate hash of ID, secret, challenge */
                lwip_md5_init(&md5_ctx);
                lwip_md5_starts(&md5_ctx);
                challenge_id = PPPOL2TP_MESSAGETYPE_SCCRP;
                lwip_md5_update(&md5_ctx, &challenge_id, 1);
                lwip_md5_update(&md5_ctx, l2tp->secret, l2tp->secret_len);
                lwip_md5_update(&md5_ctx, l2tp->secret_rv, sizeof(l2tp->secret_rv));
                lwip_md5_finish(&md5_ctx, md5_hash);
                lwip_md5_free(&md5_ctx);
                if ( memcmp(inp, md5_hash, sizeof(md5_hash)) )
                {
                    PPPDEBUG(LOG_DEBUG, ("pppol2tp: Received challenge response from peer and secret key do not match\n"));
                    pppol2tp_abort_connect(l2tp);
                    return;
                }
                goto skipavp;
#endif /* PPPOL2TP_AUTH_SUPPORT */
            default:
                break;
            }
            break;
        /* Incoming Call Reply */
        case PPPOL2TP_MESSAGETYPE_ICRP:
            switch (attributetype)
            {
            case PPPOL2TP_AVPTYPE_SESSIONID:
                if (avplen != sizeof(l2tp->source_session_id) )
                {
                    PPPDEBUG(LOG_DEBUG, ("pppol2tp: AVP Assign session ID length check failed\n"));
                    return;
                }
                GETSHORT(l2tp->source_session_id, inp);
                PPPDEBUG(LOG_DEBUG, ("pppol2tp: Assigned session ID %"U16_F"\n", l2tp->source_session_id));
                goto nextavp;
            default:
                break;
            }
            break;
        default:
            break;
        }

skipavp:
        INCPTR(avplen, inp);
nextavp:
        /* printf("AVP Found, vendor=%d, attribute=%d, len=%d\n", vendorid, attributetype, avplen); */
        /* next AVP */
        if (pbuf_header(p, -(s16_t)(avplen + sizeof(avpflags) + sizeof(vendorid) + sizeof(attributetype)) ) != 0)
        {
            return;
        }
    }

    switch(messagetype)
    {
    /* Start Control Connection Reply */
    case PPPOL2TP_MESSAGETYPE_SCCRP:
        do
        {
            l2tp->remote_session_id = magic();
        }
        while(l2tp->remote_session_id == 0);
        l2tp->tunnel_port = port; /* LNS server might have chosen its own local port */
        l2tp->icrq_retried = 0;
        l2tp->phase = PPPOL2TP_STATE_ICRQ_SENT;
        l2tp->our_ns++;
        if ((err = pppol2tp_send_scccn(l2tp, l2tp->our_ns)) != 0)
        {
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: failed to send SCCCN, error=%d\n", err));
        }
        l2tp->our_ns++;
        if ((err = pppol2tp_send_icrq(l2tp, l2tp->our_ns)) != 0)
        {
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: failed to send ICRQ, error=%d\n", err));
        }
        sys_untimeout(pppol2tp_timeout, l2tp);
        sys_timeout(PPPOL2TP_CONTROL_TIMEOUT, pppol2tp_timeout, l2tp);
        break;
    /* Incoming Call Reply */
    case PPPOL2TP_MESSAGETYPE_ICRP:
        l2tp->iccn_retried = 0;
        l2tp->phase = PPPOL2TP_STATE_ICCN_SENT;
        l2tp->our_ns++;
        ppp_start(l2tp->ppp); /* notify upper layers */
        if ((err = pppol2tp_send_iccn(l2tp, l2tp->our_ns)) != 0)
        {
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: failed to send ICCN, error=%d\n", err));
        }
        sys_untimeout(pppol2tp_timeout, l2tp);
        sys_timeout(PPPOL2TP_CONTROL_TIMEOUT, pppol2tp_timeout, l2tp);
        break;
    /* Unhandled packet, send ZLB ACK */
    default:
        goto send_zlb;
    }
    return;

send_zlb:
    pppol2tp_send_zlb(l2tp, l2tp->our_ns);
    return;
packet_too_short:
    PPPDEBUG(LOG_DEBUG, ("pppol2tp: packet too short: %d\n", p->len));
}

/* L2TP Timeout handler */
static void pppol2tp_timeout(void *arg)
{
    pppol2tp_pcb *l2tp = (pppol2tp_pcb *)arg;
    err_t err;
    u32_t retry_wait;

    PPPDEBUG(LOG_DEBUG, ("pppol2tp: timeout\n"));

    switch (l2tp->phase)
    {
    case PPPOL2TP_STATE_SCCRQ_SENT:
        /* backoff wait */
        if (l2tp->sccrq_retried < 0xff)
        {
            l2tp->sccrq_retried++;
        }
        if (!l2tp->ppp->settings.persist && l2tp->sccrq_retried >= PPPOL2TP_MAXSCCRQ)
        {
            pppol2tp_abort_connect(l2tp);
            return;
        }
        retry_wait = LWIP_MIN(PPPOL2TP_CONTROL_TIMEOUT * l2tp->sccrq_retried, PPPOL2TP_SLOW_RETRY);
        PPPDEBUG(LOG_DEBUG, ("pppol2tp: sccrq_retried=%d\n", l2tp->sccrq_retried));
        if ((err = pppol2tp_send_sccrq(l2tp)) != 0)
        {
            l2tp->sccrq_retried--;
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: failed to send SCCRQ, error=%d\n", err));
        }
        sys_timeout(retry_wait, pppol2tp_timeout, l2tp);
        break;

    case PPPOL2TP_STATE_ICRQ_SENT:
        l2tp->icrq_retried++;
        if (l2tp->icrq_retried >= PPPOL2TP_MAXICRQ)
        {
            pppol2tp_abort_connect(l2tp);
            return;
        }
        PPPDEBUG(LOG_DEBUG, ("pppol2tp: icrq_retried=%d\n", l2tp->icrq_retried));
        if (l2tp->peer_nr <= l2tp->our_ns - 1)  /* the SCCCN was not acknowledged */
        {
            if ((err = pppol2tp_send_scccn(l2tp, l2tp->our_ns - 1)) != 0)
            {
                l2tp->icrq_retried--;
                PPPDEBUG(LOG_DEBUG, ("pppol2tp: failed to send SCCCN, error=%d\n", err));
                sys_timeout(PPPOL2TP_CONTROL_TIMEOUT, pppol2tp_timeout, l2tp);
                break;
            }
        }
        if ((err = pppol2tp_send_icrq(l2tp, l2tp->our_ns)) != 0)
        {
            l2tp->icrq_retried--;
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: failed to send ICRQ, error=%d\n", err));
        }
        sys_timeout(PPPOL2TP_CONTROL_TIMEOUT, pppol2tp_timeout, l2tp);
        break;

    case PPPOL2TP_STATE_ICCN_SENT:
        l2tp->iccn_retried++;
        if (l2tp->iccn_retried >= PPPOL2TP_MAXICCN)
        {
            pppol2tp_abort_connect(l2tp);
            return;
        }
        PPPDEBUG(LOG_DEBUG, ("pppol2tp: iccn_retried=%d\n", l2tp->iccn_retried));
        if ((err = pppol2tp_send_iccn(l2tp, l2tp->our_ns)) != 0)
        {
            l2tp->iccn_retried--;
            PPPDEBUG(LOG_DEBUG, ("pppol2tp: failed to send ICCN, error=%d\n", err));
        }
        sys_timeout(PPPOL2TP_CONTROL_TIMEOUT, pppol2tp_timeout, l2tp);
        break;

    default:
        return;  /* all done, work in peace */
    }
}

/* Connection attempt aborted */
static void pppol2tp_abort_connect(pppol2tp_pcb *l2tp)
{
    PPPDEBUG(LOG_DEBUG, ("pppol2tp: could not establish connection\n"));
    l2tp->phase = PPPOL2TP_STATE_INITIAL;
    ppp_link_failed(l2tp->ppp); /* notify upper layers */
}

/* Initiate a new tunnel */
static err_t pppol2tp_send_sccrq(pppol2tp_pcb *l2tp)
{
    struct pbuf *pb;
    u8_t *p;
    u16_t len;

    /* calculate UDP packet length */
    len = 12 + 8 + 8 + 10 + 10 + 6 + sizeof(PPPOL2TP_HOSTNAME) - 1 + 6 + sizeof(PPPOL2TP_VENDORNAME) - 1 + 8 + 8;
#if PPPOL2TP_AUTH_SUPPORT
    if (l2tp->secret != NULL)
    {
        len += 6 + sizeof(l2tp->secret_rv);
    }
#endif /* PPPOL2TP_AUTH_SUPPORT */

    /* allocate a buffer */
    pb = pbuf_alloc(PBUF_TRANSPORT, len, PBUF_RAM);
    if (pb == NULL)
    {
        return ERR_MEM;
    }
    LWIP_ASSERT("pb->tot_len == pb->len", pb->tot_len == pb->len);

    p = (u8_t *)pb->payload;
    /* fill in pkt */
    /* L2TP control header */
    PUTSHORT(PPPOL2TP_HEADERFLAG_CONTROL_MANDATORY, p);
    PUTSHORT(len, p); /* Length */
    PUTSHORT(0, p); /* Tunnel Id */
    PUTSHORT(0, p); /* Session Id */
    PUTSHORT(0, p); /* NS Sequence number - to peer */
    PUTSHORT(0, p); /* NR Sequence number - expected for peer */

    /* AVP - Message type */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_MESSAGE, p); /* Attribute type: Message Type */
    PUTSHORT(PPPOL2TP_MESSAGETYPE_SCCRQ, p); /* Attribute value: Message type: SCCRQ */

    /* AVP - L2TP Version */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_VERSION, p); /* Attribute type: Version */
    PUTSHORT(PPPOL2TP_VERSION, p); /* Attribute value: L2TP Version */

    /* AVP - Framing capabilities */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 10, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_FRAMINGCAPABILITIES, p); /* Attribute type: Framing capabilities */
    PUTLONG(PPPOL2TP_FRAMINGCAPABILITIES, p); /* Attribute value: Framing capabilities */

    /* AVP - Bearer capabilities */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 10, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_BEARERCAPABILITIES, p); /* Attribute type: Bearer capabilities */
    PUTLONG(PPPOL2TP_BEARERCAPABILITIES, p); /* Attribute value: Bearer capabilities */

    /* AVP - Host name */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 6 + sizeof(PPPOL2TP_HOSTNAME) - 1, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_HOSTNAME, p); /* Attribute type: Hostname */
    MEMCPY(p, PPPOL2TP_HOSTNAME, sizeof(PPPOL2TP_HOSTNAME) - 1); /* Attribute value: Hostname */
    INCPTR(sizeof(PPPOL2TP_HOSTNAME) - 1, p);

    /* AVP - Vendor name */
    PUTSHORT(6 + sizeof(PPPOL2TP_VENDORNAME) - 1, p); /* len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_VENDORNAME, p); /* Attribute type: Vendor name */
    MEMCPY(p, PPPOL2TP_VENDORNAME, sizeof(PPPOL2TP_VENDORNAME) - 1); /* Attribute value: Vendor name */
    INCPTR(sizeof(PPPOL2TP_VENDORNAME) - 1, p);

    /* AVP - Assign tunnel ID */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_TUNNELID, p); /* Attribute type: Tunnel ID */
    PUTSHORT(l2tp->remote_tunnel_id, p); /* Attribute value: Tunnel ID */

    /* AVP - Receive window size */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_RECEIVEWINDOWSIZE, p); /* Attribute type: Receive window size */
    PUTSHORT(PPPOL2TP_RECEIVEWINDOWSIZE, p); /* Attribute value: Receive window size */

#if PPPOL2TP_AUTH_SUPPORT
    /* AVP - Challenge */
    if (l2tp->secret != NULL)
    {
        PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 6 + sizeof(l2tp->secret_rv), p); /* Mandatory flag + len field */
        PUTSHORT(0, p); /* Vendor ID */
        PUTSHORT(PPPOL2TP_AVPTYPE_CHALLENGE, p); /* Attribute type: Challenge */
        MEMCPY(p, l2tp->secret_rv, sizeof(l2tp->secret_rv)); /* Attribute value: Random vector */
        INCPTR(sizeof(l2tp->secret_rv), p);
    }
#endif /* PPPOL2TP_AUTH_SUPPORT */

    return pppol2tp_udp_send(l2tp, pb);
}

/* Complete tunnel establishment */
static err_t pppol2tp_send_scccn(pppol2tp_pcb *l2tp, u16_t ns)
{
    struct pbuf *pb;
    u8_t *p;
    u16_t len;

    /* calculate UDP packet length */
    len = 12 + 8;
#if PPPOL2TP_AUTH_SUPPORT
    if (l2tp->send_challenge)
    {
        len += 6 + sizeof(l2tp->challenge_hash);
    }
#endif /* PPPOL2TP_AUTH_SUPPORT */

    /* allocate a buffer */
    pb = pbuf_alloc(PBUF_TRANSPORT, len, PBUF_RAM);
    if (pb == NULL)
    {
        return ERR_MEM;
    }
    LWIP_ASSERT("pb->tot_len == pb->len", pb->tot_len == pb->len);

    p = (u8_t *)pb->payload;
    /* fill in pkt */
    /* L2TP control header */
    PUTSHORT(PPPOL2TP_HEADERFLAG_CONTROL_MANDATORY, p);
    PUTSHORT(len, p); /* Length */
    PUTSHORT(l2tp->source_tunnel_id, p); /* Tunnel Id */
    PUTSHORT(0, p); /* Session Id */
    PUTSHORT(ns, p); /* NS Sequence number - to peer */
    PUTSHORT(l2tp->peer_ns + 1, p); /* NR Sequence number - expected for peer */

    /* AVP - Message type */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_MESSAGE, p); /* Attribute type: Message Type */
    PUTSHORT(PPPOL2TP_MESSAGETYPE_SCCCN, p); /* Attribute value: Message type: SCCCN */

#if PPPOL2TP_AUTH_SUPPORT
    /* AVP - Challenge response */
    if (l2tp->send_challenge)
    {
        PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 6 + sizeof(l2tp->challenge_hash), p); /* Mandatory flag + len field */
        PUTSHORT(0, p); /* Vendor ID */
        PUTSHORT(PPPOL2TP_AVPTYPE_CHALLENGERESPONSE, p); /* Attribute type: Challenge response */
        MEMCPY(p, l2tp->challenge_hash, sizeof(l2tp->challenge_hash)); /* Attribute value: Computed challenge */
        INCPTR(sizeof(l2tp->challenge_hash), p);
    }
#endif /* PPPOL2TP_AUTH_SUPPORT */

    return pppol2tp_udp_send(l2tp, pb);
}

/* Initiate a new session */
static err_t pppol2tp_send_icrq(pppol2tp_pcb *l2tp, u16_t ns)
{
    struct pbuf *pb;
    u8_t *p;
    u16_t len;
    u32_t serialnumber;

    /* calculate UDP packet length */
    len = 12 + 8 + 8 + 10;

    /* allocate a buffer */
    pb = pbuf_alloc(PBUF_TRANSPORT, len, PBUF_RAM);
    if (pb == NULL)
    {
        return ERR_MEM;
    }
    LWIP_ASSERT("pb->tot_len == pb->len", pb->tot_len == pb->len);

    p = (u8_t *)pb->payload;
    /* fill in pkt */
    /* L2TP control header */
    PUTSHORT(PPPOL2TP_HEADERFLAG_CONTROL_MANDATORY, p);
    PUTSHORT(len, p); /* Length */
    PUTSHORT(l2tp->source_tunnel_id, p); /* Tunnel Id */
    PUTSHORT(0, p); /* Session Id */
    PUTSHORT(ns, p); /* NS Sequence number - to peer */
    PUTSHORT(l2tp->peer_ns + 1, p); /* NR Sequence number - expected for peer */

    /* AVP - Message type */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_MESSAGE, p); /* Attribute type: Message Type */
    PUTSHORT(PPPOL2TP_MESSAGETYPE_ICRQ, p); /* Attribute value: Message type: ICRQ */

    /* AVP - Assign session ID */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_SESSIONID, p); /* Attribute type: Session ID */
    PUTSHORT(l2tp->remote_session_id, p); /* Attribute value: Session ID */

    /* AVP - Call Serial Number */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 10, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_CALLSERIALNUMBER, p); /* Attribute type: Serial number */
    serialnumber = magic();
    PUTLONG(serialnumber, p); /* Attribute value: Serial number */

    return pppol2tp_udp_send(l2tp, pb);
}

/* Complete tunnel establishment */
static err_t pppol2tp_send_iccn(pppol2tp_pcb *l2tp, u16_t ns)
{
    struct pbuf *pb;
    u8_t *p;
    u16_t len;

    /* calculate UDP packet length */
    len = 12 + 8 + 10 + 10;

    /* allocate a buffer */
    pb = pbuf_alloc(PBUF_TRANSPORT, len, PBUF_RAM);
    if (pb == NULL)
    {
        return ERR_MEM;
    }
    LWIP_ASSERT("pb->tot_len == pb->len", pb->tot_len == pb->len);

    p = (u8_t *)pb->payload;
    /* fill in pkt */
    /* L2TP control header */
    PUTSHORT(PPPOL2TP_HEADERFLAG_CONTROL_MANDATORY, p);
    PUTSHORT(len, p); /* Length */
    PUTSHORT(l2tp->source_tunnel_id, p); /* Tunnel Id */
    PUTSHORT(l2tp->source_session_id, p); /* Session Id */
    PUTSHORT(ns, p); /* NS Sequence number - to peer */
    PUTSHORT(l2tp->peer_ns + 1, p); /* NR Sequence number - expected for peer */

    /* AVP - Message type */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_MESSAGE, p); /* Attribute type: Message Type */
    PUTSHORT(PPPOL2TP_MESSAGETYPE_ICCN, p); /* Attribute value: Message type: ICCN */

    /* AVP - Framing type */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 10, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_FRAMINGTYPE, p); /* Attribute type: Framing type */
    PUTLONG(PPPOL2TP_FRAMINGTYPE, p); /* Attribute value: Framing type */

    /* AVP - TX Connect speed */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 10, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_TXCONNECTSPEED, p); /* Attribute type: TX Connect speed */
    PUTLONG(PPPOL2TP_TXCONNECTSPEED, p); /* Attribute value: TX Connect speed */

    return pppol2tp_udp_send(l2tp, pb);
}

/* Send a ZLB ACK packet */
static err_t pppol2tp_send_zlb(pppol2tp_pcb *l2tp, u16_t ns)
{
    struct pbuf *pb;
    u8_t *p;
    u16_t len;

    /* calculate UDP packet length */
    len = 12;

    /* allocate a buffer */
    pb = pbuf_alloc(PBUF_TRANSPORT, len, PBUF_RAM);
    if (pb == NULL)
    {
        return ERR_MEM;
    }
    LWIP_ASSERT("pb->tot_len == pb->len", pb->tot_len == pb->len);

    p = (u8_t *)pb->payload;
    /* fill in pkt */
    /* L2TP control header */
    PUTSHORT(PPPOL2TP_HEADERFLAG_CONTROL_MANDATORY, p);
    PUTSHORT(len, p); /* Length */
    PUTSHORT(l2tp->source_tunnel_id, p); /* Tunnel Id */
    PUTSHORT(0, p); /* Session Id */
    PUTSHORT(ns, p); /* NS Sequence number - to peer */
    PUTSHORT(l2tp->peer_ns + 1, p); /* NR Sequence number - expected for peer */

    return pppol2tp_udp_send(l2tp, pb);
}

/* Send a StopCCN packet */
static err_t pppol2tp_send_stopccn(pppol2tp_pcb *l2tp, u16_t ns)
{
    struct pbuf *pb;
    u8_t *p;
    u16_t len;

    /* calculate UDP packet length */
    len = 12 + 8 + 8 + 8;

    /* allocate a buffer */
    pb = pbuf_alloc(PBUF_TRANSPORT, len, PBUF_RAM);
    if (pb == NULL)
    {
        return ERR_MEM;
    }
    LWIP_ASSERT("pb->tot_len == pb->len", pb->tot_len == pb->len);

    p = (u8_t *)pb->payload;
    /* fill in pkt */
    /* L2TP control header */
    PUTSHORT(PPPOL2TP_HEADERFLAG_CONTROL_MANDATORY, p);
    PUTSHORT(len, p); /* Length */
    PUTSHORT(l2tp->source_tunnel_id, p); /* Tunnel Id */
    PUTSHORT(0, p); /* Session Id */
    PUTSHORT(ns, p); /* NS Sequence number - to peer */
    PUTSHORT(l2tp->peer_ns + 1, p); /* NR Sequence number - expected for peer */

    /* AVP - Message type */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_MESSAGE, p); /* Attribute type: Message Type */
    PUTSHORT(PPPOL2TP_MESSAGETYPE_STOPCCN, p); /* Attribute value: Message type: StopCCN */

    /* AVP - Assign tunnel ID */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_TUNNELID, p); /* Attribute type: Tunnel ID */
    PUTSHORT(l2tp->remote_tunnel_id, p); /* Attribute value: Tunnel ID */

    /* AVP - Result code */
    PUTSHORT(PPPOL2TP_AVPHEADERFLAG_MANDATORY + 8, p); /* Mandatory flag + len field */
    PUTSHORT(0, p); /* Vendor ID */
    PUTSHORT(PPPOL2TP_AVPTYPE_RESULTCODE, p); /* Attribute type: Result code */
    PUTSHORT(PPPOL2TP_RESULTCODE, p); /* Attribute value: Result code */

    return pppol2tp_udp_send(l2tp, pb);
}

static err_t pppol2tp_xmit(pppol2tp_pcb *l2tp, struct pbuf *pb)
{
    u8_t *p;

    /* make room for L2TP header - should not fail */
    if (pbuf_header(pb, (s16_t)PPPOL2TP_OUTPUT_DATA_HEADER_LEN) != 0)
    {
        /* bail out */
        PPPDEBUG(LOG_ERR, ("pppol2tp: pppol2tp_pcb: could not allocate room for L2TP header\n"));
        LINK_STATS_INC(link.lenerr);
        pbuf_free(pb);
        return ERR_BUF;
    }

    p = (u8_t *)pb->payload;
    PUTSHORT(PPPOL2TP_HEADERFLAG_DATA_MANDATORY, p);
    PUTSHORT(l2tp->source_tunnel_id, p); /* Tunnel Id */
    PUTSHORT(l2tp->source_session_id, p); /* Session Id */

    return pppol2tp_udp_send(l2tp, pb);
}

static err_t pppol2tp_udp_send(pppol2tp_pcb *l2tp, struct pbuf *pb)
{
    err_t err;
    if (l2tp->netif)
    {
        err = udp_sendto_if(l2tp->udp, pb, &l2tp->remote_ip, l2tp->tunnel_port, l2tp->netif);
    }
    else
    {
        err = udp_sendto(l2tp->udp, pb, &l2tp->remote_ip, l2tp->tunnel_port);
    }
    pbuf_free(pb);
    return err;
}

#endif /* PPP_SUPPORT && PPPOL2TP_SUPPORT */
